Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>
Hmm, maybe do this check in the uber test and just print a warning with success rather than failing the test so it still works on platforms where it isn't supported. (I know we are assuming that ENOTSUP works correctly but that should be okay I think).